To increase production, tillage practices have become more sophisticated in the past century, including increasing the depth of tillage and numerous specialized implements to create a desirable seed bed and control unwanted vegetation. With the evolution and use of large agricultural machines came detrimental impacts to soil below the normal depth of tillage. These impacts included compaction, shearing of soil during wheel-slip, and rutting/sinking of tires into the soil, which altered natural soil structure impeded the flow of water and air into and through the soil, and the penetration of the soil by roots. As the size of the machines increased, so did the depth and severity of the adverse impacts to soil by trafficking. The subsoil can take several decades, and sometimes more than a century to recover from the adverse impacts due to a single period of trafficking.
The temporary use of agricultural, forest, and range lands for industrial purposes such as oil and gas exploration and extraction, mining for minerals, and the facilities and infrastructure to access these developments can also severely impact soil productivity and the type and sustainability of the plant community returned to industrial sites once they are reclaimed and returned to their original owners. Hence, deep tillage of these types of sites is probably more important than it is to lands used by modern agriculture. In the majority of the cases, the implements used to till reclaimed soil are larger versions of those used in modern agriculture, which generally have developed from a common scientific basis. As a result, the depth of tillage is relatively shallow, and the soil condition when they are effective in loosening soil is a narrow window, which in some climates may not exist.
A variety of inventions for tillage of soil below the normal plough layer tilled in agricultural practice have been developed, include shanks with special points, tines, sweeps, or wings, which are designed to efficiently move through the soil while loosening the soil above the soil engaging, leading edge of the implement. The primary forces imparted to soil by deep subsoiling inventions focus most of the energy in the forward and vertical axes; only a few such as paraploughs for example exert a lateral force on the soil as well. Regardless, the principle method of tillage is to use a relatively thin narrow blade to slice through the soil as a specific depth and raise the soil a short distance before the soil is allowed to fall over the backside of the blade. The fall from the back of the blade causes a tensile failure of the soil because the tensile strength of soil can be very low compared to its compressive or shear strength. When ripper ploughs, include wings or sweeps on the shank, the included angle is generally much greater than 90 degrees, which maintains most of the tillage energy concentrated in the direction of travel and vertical planes. These types of equipment are highly effective to depths of 40 to 50 cm when the soils have only been compacted but still retain much of its natural structure and the entire soil profile is relatively dry.
The natural structure of soil creates planes in soil which become the most likely zones of fracturing during tillage to produce tensile failure in soil. Severe compaction and shearing of soil during slip of wheels and tracks of machines working near their maximum capability destroy, at least some if not all of the natural structure of soil. The destruction of soil structure creates a homogenous blend of soil particles, which is commonly referred to as massive soil, or a soil with massive structure. These conditions are common during reclamation of industrial sites when ground-engaging equipment are used to move wetter soil.
Massive structure of cohesive soil and/or wetter soil severely limits the effective depth that conventional tillage implements can till subsoil. Several factors contribute to this deficiency. Deep tillage is ineffective under these conditions because of the primary reliance on tensile failure in the two dimensional planes of direction of travel and vertical lift. In these conditions, passive pressure form of earth pressure theory applies, or only a small zone of active press develops immediately around a shank. As a result of passive pressures in the soil around the ground engaging portions of a subsoiler, the fracture plane from its outer edge to the soil surface is normally an angle less than 45 degrees from vertical. Hence, the depth that the soil engaging portion of a subsoiler is operated is nearly always greater than the width of the soil loosened at the surface. As soil becomes more massive and/or wetter, the more likely the soil will generally deform and flow around the shank and ground-engaging portions of subsoilers. In these situations, it is not uncommon for subsoilers to only form slits in the soil around the shank and ground engaging tines, wings, and sweeps, and the volume of soil is limited to a small “V-shaped” area of soil immediately around the shank. Numerous examples of these types of soil fracturing patterns and failure of conventional equipment to till massive and/or wetter soil can be found in the soil and reclamation scientific literature.
Relatively recent research in unsaturated soil strength is also providing additional information on why conventional subsoilers have a narrow window of effectiveness in cohesive, finer-textured soils. These soils become more brittle as they dry and have higher soil strength than the same soil when wet. Drying can also cause shrinkage which can separate soil structural units. These factors cause the soil to reach maximum strength and fail at relatively low strain. Strain is defined as the ratio of change in volume or lateral displacement of soil relative to its total volume or length of the soil unit. These attributes of soil strength contribute to the relatively easy failure of soil by tensile failure when tilled with wings or sweeps on subsoilers with low angles of lift. As soil moisture increases, the amount of strain or soil displacement required to fail increases. In dry soil, a soil may fail at a strain of only a few percent, whereas wet soil can be deformed by a strain of between 10 to 20 percent before it fails. Hence, wet soil requires a much higher rate of strain to fracture into clods.
U.S. Pat. No. 5,415,236 (Williams) entitled “Subsoiler Having Rearwardly Disposed Soil Fracturing Structure” describes a subsoiler having shank assemblies that cut a slice in the soil without causing an eruption of the soil in a turbulent manner ahead of the shank line.